= Hubble constant
{c}
{title2=$H_0$}
The <Hubble constant> is the present value $H_0=H(t_0)$ of the <Hubble parameter>. The word "constant" fixes the epoch; it does not mean $H(t)$ stays unchanged throughout <cosmic time>. It sets the present instantaneous proportionality in <Hubble's law> and the approximate nearby observed <redshift>-distance relation.
For illustration, stipulate $H_0=70\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$; this is an example input, not a claim about a measured consensus value. A million <parsecs> is $3.085677581\ldots\times10^{19}\,\mathrm{km}$, giving
$$
H_0\simeq2.26855\times10^{-18}\,\mathrm{s^{-1}},\qquad
H_0^{-1}\simeq4.40811\times10^{17}\,\mathrm s\simeq13.97\,\mathrm{Gyr}.
$$
Here a year means $365.25$ days of $86400\,\mathrm s$, and $\mathrm{Gyr}$ denotes a billion such years. The <parsec> conversion follows https://arxiv.org/pdf/1510.06262[IAU 2015 Resolution B2, note 4]{ref}. This <Hubble time> is a present expansion timescale; the <age of an FLRW universe> also depends on the earlier evolution of the <scale factor> and need not equal $H_0^{-1}$.
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